Conductive carrier particles that support fine particles of platinum or platinum alloys are useful as electrode catalysts that catalyze hydrogen oxidation reactions at the hydrogen electrode and oxygen reduction reactions at the oxygen electrode of a fuel cell. Accordingly, research of such carrier particles aimed at practical application thereof has been actively conducted.
However, several technical difficulties arise with the use of platinum fine particles for electrode catalysts. For example, platinum fine particles are dissolved due to electric potential changes, and activity thereof is lowered due to adsorption of carbon monoxide at the particle surfaces. Thus, the durability of electrode catalysts using platinum fine particles is not always satisfactory.
In order to overcome such drawback, development of techniques for improving the durability of platinum fine particles as catalysts via surface modification with gold has heretofore been conducted.
Patent Documents 1 and 2 each disclose a technique for modifying a platinum-supporting carbon catalyst via gold deposition so as to enhance the power generation efficiency of a platinum-supporting carbon catalyst. Specifically, Patent Documents 1 and 2 each disclose a method of adding a reducing agent (NaBH4) dropwise to a mixed solution of a platinum-supporting carbon catalyst and a gold salt to reduce and deposit gold and thus to cover platinum with gold.
Patent Document 3 discloses a method for producing an exhaust gas purging catalyst comprising adding a reducing agent (Na2S2O3) dropwise to a mixed solution of a metal oxide that supports platinum particles and a gold salt at a solution temperature of 60° C. to cover the platinum particle surface with gold.
Patent Document 4 discloses a technique for covering and protecting at least part of the surfaces of platinum fine particles supported on conductive carrier particles with a thin gold film. The method for forming a thin gold film disclosed by Patent Document 4 is as follows. At the outset, conductive carrier particles that support platinum fine particles are placed on the electrode, the resultant is soaked in an aqueous solution of roughly 50 mM CuSO4/0.10M H2SO4, and an adequate reduction potential is appled to the electrode to form a monatomic copper layer on the platinum fine particle surfaces. This process is referred to as “underpotential deposition (UPD).” In the UPD process, a potential slightly more positive than a copper reduction potential (i.e., 0.337 V at pH 2 or less comparison with a standard hydrogen electrode)) is applied to deposit a monatomic copper layer on the platinum fine particle surface. Subsequently, the electrode is soaked in a solution of a gold salt, and a copper monatomic layer having a reduction potential lower than that of gold is displaced with a gold monatomic layer via spontaneous redox displacement. Thus, at least part of the platinum fine particle surface is covered with a gold monatomic layer.